1. Field of the Invention
The present invention generally relates to an optical scanner and an image forming apparatus, such as a laser printer, a digital copier, a laser facsimile machine, etc., that includes the optical scanner.
2. Discussion of the Background Art
In general, an electronographic image forming apparatus, for example, a digital copier, a laser printer, a facsimile machine, etc., that forms an image on a sheet of recording media includes an optical scanner (hereinafter also “optical writing unit”) configured to direct an optical beam emitted from a light source onto a surface of an image carrier according to image information, forming an electrostatic latent image thereon. Then, the electrostatic latent image is developed with toner, transferred from the image carrier onto the sheet, and fixed thereon.
The optical writing unit generally includes a polygon mirror that deflects the optical beam while rotating, multiple optical devices, such as an f-theta (θ) lens, a scanning lens, reflecting mirrors, etc., that direct the deflected optical beam onto the surface of the image carrier, and an optical detector configured to determine a writing start point. The optical detector is located on a side where scanning begins, outside an effective exposure area.
In such an image forming apparatus, a line of the optical beam scanning across the surface of the image carrier (hereinafter “scanning line”) tends to curve. That is, the image forming apparatus includes devices, such as a motor for driving the polygon mirror, that generate heat and can cause the optical devices to deform thermally. Such thermal deformation, curvature of imaging surfaces of the optical devices, assembly error, installation error of the image carrier, etc., can cause the main scanning line to curve.
Particularly, in multicolor image forming, in which different single-color images are superimposed one on another, if each of multiple scanning lines curves differently from the other, the final image will have color deviation, that is, the superimposed single-color images are not aligned with each other in the multicolor image.
In order to reduce curvature of the scanning line (hereinafter “scanning line curvature”), several approaches involving curving the reflecting mirror that is an optical device have been suggested.
In one known method, multiple reflecting mirrors configured to reflect an optical beam are disposed in an optical path, and the reflecting mirror that is closest to the surface to be scanned by the optical beam has a surface concave or convex in a normal direction so as to correct the scanning line curvature on the scan to be scanned.
In another known method, a reflecting mirror disposed in an optical path is deformed in a normal direction with respect to a reflecting surface thereof so as to correct the scanning line curvature.
However, the methods described above do not disclose a means nor technique to regulate the relative positions of the reflecting mirror and a holder holding the reflecting mirror so as to maintain the corrected scanning line.
For example, even if the reflecting mirrors are adjusted with each other preliminarily so as to align the scanning line curvature among them, the reflecting mirrors can be misaligned if the optical writing unit receives physical or thermal impact at a user's site, or is shaken during transport. In such a case, adjustment of the scanning lines can be disturbed.
Therefore, it is necessary to maintain the adjusted scanning lines after the scanning lines are adjusted during initial adjustment or factory adjustment.